I think you brood over rejections early in your career. After a while, you figure they happen. They may still sting, but you know you aren't going to figure out what was in the editor's mind.
Thirty years ago, Robert Silverberg rejected my story "The Warlord of Saturn's Moons." It was published by New Worlds and reprinted in Pamela Sargent's Women of Wonder series and the Norton Book of Science Fiction, edited by Brian Attebery and Ursula K. LeGuin.
Somewhere along the line, Silverberg told a friend of mine he would have bought the story, if he had ever seen it. I think I still have Silverberg's rejection note. He said he liked the story, but it wasn't science fiction. He then rejected the next story I sent him, saying it was science fiction, but he didn't like it. Damon Knight published that one in his Orbit series.
Gardner Dozois rejected my story "The Garden," when I sent it to Asimov's. George Zebrowski bought it for an anthology, and Gardner put it in his Best of the Year collection.
David Gerrold once sent me -- decades ago -- a form rejection which was intended to be comic. It said, if I remember correctly, if his mother were dying and the only way he could save her was to accept this story, he wouldn't.
I returned the rejection to him, explaining that it was not acceptable; and he sent me an apology. He must have been very young then, and it was meant to be a joke. But I really did not think it was acceptable.